experts may have another (better!) suggestion, but here’s mine:
the white material is the very fine epoxy sanding dust nestled into low spots in the fiberglass cloth. I had that in a few places. I assumed that to get rid of the white spots I would have to sand almost all the way through the ‘glass which I didn’t want to do. so I used a stiff toothbrush soaked in denatured alcohol and was able to dislodge almost all the sanding dust from the pits.
ken